1. Field of the Invention
The present invention relates generally to the field of medical devices, and more particularly, to the encapsulation of stents.
2. Description of Related Art
Stents and similar endoluminal devices are currently used by medical practitioners to treat tubular body vessels or ducts that become so narrowed (stenosed) that flow of blood or other biological fluids is restricted. Such narrowing (stenosis) occurs, for example, as a result of the disease process known as arteriosclerosis. While stents are most often used to xe2x80x9cprop openxe2x80x9d blood vessels, they can also be used to reinforce collapsed or narrowed tubular structures in the respiratory system, the reproductive system, bile or liver ducts or any other tubular body structure. However, stents are generally mesh-like so that endothelial and other tissues can grow through the openings resulting in restenosis of the vessel.
Polytetrafluoroethylene (PTFE) has proven unusually advantageous as a material from which to fabricate blood vessel grafts or prostheses, tubular structures that can be used to replace damaged or diseased vessels. This is partially because PTFE is extremely biocompatible causing little or no immunogenic reaction when placed within the human body. This is also because in its preferred form, expanded PTFE (ePTFE), the material is light and porous and is readily colonized by living cells so that it becomes a permanent part of the body. The process of making ePTFE of vascular graft grade is well known to one of ordinary skill in the art. Suffice it to say that the critical step in this process is the expansion of PTFE into ePTFE. This expansion represents a controlled longitudinal stretching in which the PTFE is stretched to several hundred percent of its original length.
Apart from use of stents within the circulatory system, stents have proven to be useful in dealing with various types of liver disease in which the main bile duct becomes scarred or otherwise blocked by neoplastic growths, etc. Such blockage prevents or retards flow of bile into the intestine and can result in serious liver damage. Because the liver is responsible for removing toxins from the blood stream, is the primary site for the breakdown of circulating blood cells and is also the source of vital blood clotting factors, blockage of the bile duct can lead to fatal complications. A popular type of stent for use in the biliary duct has been one formed from a shape memory alloy (e.g., nitinol) partially because such stents can be reduced to a very low profile and remain flexible for insertion through the sharp bend of the bile duct while being, self-expandable and capable of exerting a constant radial force to the duct wall.
Cellular infiltration through stents can be prevented by enclosing the stents with ePTFE. Early attempts to produce a stent covered by ePTFE focused around use of adhesives or physical attachment such as suturing. However, such methods are far from ideal and suturing, in particular, is very labor intensive. More recently methods have been developed for encapsulating a stent between two tubular ePTFE members whereby the ePTFE of one-member touches and bonds with the ePTFE of the other member through the mesh opening in the stent. However, such a mnonolithically encapsulated stent may tend to be rather inflexible. Therefore, there is a need for a stent covered to prevent cellular infiltration and yet still flexible to ensure ease of insertion and deployment and to accommodate extreme anatomical curves.
The present invention is directed to covered stents wherein flexibility of the stent is retained, despite the use of encapsulation techniques. Encapsulation refers to the lamination of a stent between an inner and an outer layer of a plastic material. Compared to a fully encapsulated stent enhanced flexibility can be achieved by encapsulating limited regions of the stent, while leaving a significant portion of the stentxe2x80x94usually a middle portionxe2x80x94covered by a single layer of the plastic material. In this way the limited encapsulation fixes the plastic covering onto the stent with no need for sutures or similar labor intensive mechanical attachments.
It is an object of this invention to provide a stent device that has improved flexibility compared to a fully encapsulated stent, yet maintains its impermeability to infiltrating tissues.
It is yet another object of this invention to provide a stent device that shows minimal profile when loaded into insertion systems and can be deployed using forces that are reduced compared to those used with fully encapsulated designs.
These and additional objects are accomplished by embedding or encapsulating only portions of the stent between two layers of biocompatible material. This is accomplished by covering either the luminal or abluminal surface of the stent with a layer of biocompatible material, preferably ePTFE, while also covering limited sections of the opposite surface of the stent with the biocompatible material, thereby fully encapsulating only the limited sections. A preferred design fully encapsulates only the end regions of the device. By leaving a middle region of the stent unencapsulated, the stent is free to flex much like a bare stent, increasing overall flexibility and reducing the necessary loading and deployment forces.
In the present invention, a stent is partially encapsulated using the configuration mentioned above. One means of accomplishing this configuration is to place rings (radial strips) of ePTFE on a mandrel at positions corresponding to each end of the stent. The stent is then placed over the mandrel and the rings in registration with the ends of the stent. Finally, the stent (supported by the mandrel) is covered on its abluminal (outside) surface by a tubular ePTFE graft. The resulting structure is then subjected to heat and pressure so that the regions containing ePTFE on both surfaces become laminated or fused together (e.g., a bond is formed). This yields a stent with substantially its entire abluminal surface covered by ePTFE. Regions near the ends of the stent are fully encapsulated (e.g., these regions are covered by ePTFE on their luminal surfaces as well). The fully encapsulated area serves to attach the abluminal covering to the stent.